Long-term thermal stability and failure mechanisms of Pyromark 2500 for high-temperature solar thermal receivers

Abstract

Pyromark 2500 is widely used as a concentrating solar power absorber coating because of its relatively high absorptance and its ease in application onto large-scale solar thermal receiver surfaces. However, it cannot withstand many years at elevated temperatures (>700 °C), degrading significantly within the first years of operation, and eventually failing unless costly maintenance procedures are carried out. This work aims to further the understanding of the reasons for Pyromark degradation and failure. Pyromark samples deposited on Inconel 625 and stainless steel 316L were analysed after undergoing extensive isothermal aging testing at 800 and 900 °C for 3000 h. Pyromark on Inconel 625 was shown to be optically stable at 800 °C for 3000 h, retaining a high figure of merit, which includes the impact of radiative losses as well as absorption, with a value of 90.7% after this time. In contrast, at 900 °C, the solar absorptance and optical performance of the coating trended consistently downward, with mechanical failure after 1200 h. Part of the optical degradation was associated with observed changes in morphology and the formation of new phases. Spallation was identified as the main physical degradation mechanism. The analysis suggested that the coating spalled due to two main factors: a reduction in bonding strength owing to micro-defects and the formation of oxide layers; and stress caused by the thermal expansion coefficient mismatch between layers. At higher temperatures, a more significant reduction in bonding strength, combined with higher thermal stresses, led to the rapid failure of the coating. • Optical and physical degradation of Pyromark is studied after long-term aging. • Pyromark on Inconel 625 was optically stable at 800 °C, while it failed at 900 °C. • New phase formation and changes in morphology impact optical performance. • Spallation failure is a result of increasing stress and reducing bonding strength. • Residual thermal stress between coating and substrate increases over aging time.